The usual scenario is that a lens with a wide focal length range (zoom)...will have compromises in image quality. If you are OK with that, then something like an 18-200mm lens might be a good fit for you. If image quality is higher on the list, you may have to make the compromise and carry more lenses.

If I were limited to one lens it would be a 35mm SMC-A Pentax. Im not sure if they made it as a macro or not but that would be a plus. I dont have one of these lenses but I am looking all over for one.